1. Field of the Invention
The present invention pertains to glide heads for determining the uniformity of the flat surfaces of rotating recording disks by sensing bumps or asperities thereon, and more particularly, it pertains to glide heads of a type which can not only determine the presence of asperities of greater than a predetermined maximum height so as to be useful in determining the necessity of further burnishing operations but also accurately map or locate the asperities on the recording disk face so as to provide disk quality control information for possible adjustment of the disk manufacturing process parameters.
2. Description of the Prior Art
The basic memory storage device of computers today is the rigid magnetic disk, or so called hard disk, which includes a thin magnetic film on both of its surfaces to which information can be written or read by tiny magnetic transducer heads suspended from actuator arms which are adapted to be moved radially across the surfaces of the disk. The magnetic information is commonly imparted to the disk in a series of concentric tracks extending radially about the disk surfaces. As the disk is rotated, information is transferred between each magnetic head and the disk with the head being spaced from the disk surface by only a very small air bearing distance so that the magnetic head does not ride directly upon and damage, or be damaged by, the disk surface. With the ever increasing demand for higher disk storage capacities, the number of magnetic tracks on the disk is increasing while the spacing thereof is decreasing, and the number of individual magnetic bits per unit distance along each track is increasing; consequently, the magnetic head must ride ever closer to the surface of the disk in order to properly write information on and read magnetic information from the disk. In today's market the flying height of such data heads over the disk has been reduced to about 1-2 microinches, and this distance is shrinking and will continue to shrink as the bit storage capacity for unit area on the disks increases. Obviously, in order to avoid collisions between the magnetic head and the disk during the reading/writing operation of the head, the disk surface relative to the flying height of the magnetic head must be flat, stable, and free from protrusions, or asperities, rising above the flat surface of the disk which asperities could cause damage to the magnetic heads and which could cause errors in writing to or reading from the magnetizable plane in the disk adjacent to the disk surface.
Magnetic disks are typically comprised of a substrate of aluminum/magnesium alloy upon which a thin layer of magnetic film is sputtered with the magnetic film being adapted to be magnetized and read by the magnetic head passed thereover. The disk is then typically overcoated with a thin layer of carbon at a thickness at about 5-250 angstroms to provide protection for the disk surface. This protective layer provides erosion resistance and damage protection from impact and is adapted to be sputtered onto the disk. It will be appreciated, however, that none of the manufacturing processes can be absolutely perfect and that asperities can develop on the disk surface, i.e., rough areas where the surface elevation will vary. Such asperities may be large enough to permit the disk to contact the magnetic head as it travels over the rapidly moving disk surface, and consequent damage to the disk can occur as mentioned above. Even if head/disk collisions do not occur, shock waves created in the magnetic head due to the change in air pressure therebetween caused by an asperity on the disk surface can result in the miswriting or misreading of magnetically encoded information on the disk surface. In view of the foregoing, it is a standard process in magnetic disk manufacture to pass a glide head over the disk surface, in a manner somewhat similar to the manner in which the magnetic heads will be passed over the surface during subsequent read/write operations, in order to test the surface for asperities in order that such asperities may be eliminated or reduced in size by subsequent burnishing operations. Furthermore, it has become conventional to use such glide heads to locate and map the location of the asperities on the disk surface in order to provide corrections in the manufacturing process, if necessary, in order to achieve a more perfectly flat and planar surface on the disk.
The glide heads are generally comprised of a slider, much like the sliders used in the conventional read/write magnetic heads, which slider is flexibly supported from a flexure arm and which includes a pair of radially spaced flight rails, or other configurations of air-bearing surfaces, that cause the slider to ride above the disk with a slight backward tilt whereby the trailing edge of the slider rides on an air bearing surface at a very close spacing to the disk surface. This slide head-to-disk surface spacing will typically be somewhat less than the spacing of the read/write magnetic head when the finished recording disk is used in data transfer operation. A piezoelectric transducer is located on the glide head, and electrical leads are provided from the transducer to appropriate external circuitry so that stresses in the piezoelectric element created by collisions or near misses between the glide head and the disk surface (i.e., rapid changes in sensed pressure) due to asperities on the surface of the disk can be monitored.
Depending on how the piezoelectric transducer is positioned on the slider, it has been conventional to look for an electrical output signal with a dominant frequency band, i.e., a natural frequency of vibration of the slider/piezoelectric transducer in a particular mode or plane due to glide head/disk surface interference. That is to say, as the glide head flies over the disk, any asperities or protrusions above the nominal disk surface elevation sufficient to cause a change in pressure at the trailing edge of the slider will induce a dominant mode of vibration in the piezoelectric transducer which can be read and interpreted in a manner so as to define the nature of the asperity. Because the sensing edge of the slider will extend for some distance radially of the rotating disk, and since the sensing edge of a slider will not be uniform due, typically, to the presence of the requisite spaced flight rails defining the same, it will be apparent that asperity/glide head interference will produce a number of different vibrational modes in the piezoelectric transducer which will occur in locations and at frequencies as determined by the size and shape of the piezoelectric element and (particularly) its location with respect to the contact point on the slider element as well as by the location and magnitude of the asperities creating the vibrations in the glide head. For example, an asperity which contacts only one flight rail of the slider element will produce a distinctly different set of vibrational modes in the transducer than a pair of asperities of a similar elevation which contact both flight rails at the same time.
The foregoing factors have created difficulties in obtaining reliable readings from the glide heads so that the exact nature and height of asperities can be determined in order to know whether the requisite "flatness" of the disk has been obtained or whether further burnishing operations are required. Even greater problems are encountered with conventional glide heads in determining the specific location and extent of asperities on the disk surface due to the various modes of vibration which are set up in the transducer element which in some cases may cancel each other out or be unrealistically combined.
One method of obtaining a more reliable and informative reading from a glide head is shown in prior U.S. Pat. No. 5,423,207 to Flechsig et al.; U.S. Pat. No. 5,450,747 to Flechsig et al.; U.S. Pat. No. 4,532,802 to Yeak-Scranton et al., and IBM Technical Disclosure Bulletin Volume 34, No. 4a, September 1991, page 459. These prior art disclosures generally show glide heads which include, not one, but a plurality of separate transducers affixed to the body of the slider element which transducers can be separately monitored with the outputs thereof being compared so as to provide a better analysis of the surface of the disk being monitored.
Another method of more reliably obtaining a transducer output or reading which accurately reflects the surface contours of the recording disk being monitored is shown in U.S. Pat. No. 5,689,064 to Kennedy et al. Here, the transducer element does not directly overlie the slider element (as is conventional in most prior art devices) but is attached to the slider only at one narrow face thereof so as to extend laterally of the slider in cantilevered fashion whereby the dominant mode or modes of vibration of the transducer element can be more accurately determined and appropriate filtering circuitry can be utilized to obtain more reliable glide head readings.